In recent years, many electronic apparatuses have a micro processor (hereinafter, referred to as CPU) as a system controller. Further, the operating frequency of the CPU has become higher, and as the operating frequency is increasing, so the maximum operating current is increasing.
Under such circumstances, in order to meet the requirement for this large current, there is proposed a multiple phase power supply system where the current supply performance has been increased by connecting plural switching power supplies in parallel. In such a power supply system, if the currents flowing in respective power supplies (phases) are not uniform, heat generation concentrates and causes destruction of the power supply system. It is necessary to precisely detect currents flowing in coils and perform a feedback control of drive signals of power transistors of respective phases so that uniform current should flow to the respective phases. In current sensing for the feedback control, high precision is required for controlling currents of the respective phases evenly.
As a method of detecting current, there is for example one described in Japanese Patent Application Laid-open Publication No. 2003-232816 (Patent Document 1).
Meanwhile, the Patent Document 1 does not take into consideration the case to be used in a switching power supply, further, it does not mention about influences of detection errors due to offset of an operational amplifier of sensing current, and both positive and negative power supplies are required for the operational amplifier.